Israel's top military intelligence analyst says Syria used chemical weapons against rebels last month. Obama has said such an attack would be a 'game changer.'

Israeli tanks sit near the Syrian border in the Golan Heights before the… (Atef Safadi, European Pressphoto…)

JERUSALEM — Israel's accusation that Syria used chemical weapons against rebels raises the prospect that Damascus crossed what President Obama has termed a "red line," but appears unlikely to overcome deep resistance of the U.S. and its allies to military involvement in the country's civil war.

Brig. Gen. Itai Brun, Israel's top military intelligence analyst, said at a security conference in Tel Aviv on Tuesday that Syria used chemical weapons, probably a sarin-based nerve agent, in attacks March 19 near Aleppo and Damascus. He said the assessment was based on reports of victims foaming at the mouth and having constricted pupils.

The disclosure followed similar assessments last week by Britain and France, which asked the United Nations to investigate.

U.S. officials say they are evaluating the reports that the beleaguered Syrian government unleashed its stockpile of chemical weapons, by some estimates the third-largest in the world. Privately, U.S. officials say they remain unconvinced by the assessments of three close allies — even that of Israel, whose regional intelligence-gathering has long been crucial to the United States.

Britain and France "did not provide conclusive evidence of chemical weapons use" in their request to the U.N., said a senior Defense official who spoke on condition of anonymity because of the sensitivity of the issue.

The Israeli military official criticized the international community for failing to respond to the Syrian attacks. "To the best of our professional understanding, the regime used lethal chemical warfare on a number of occasions during recent months," Brun said. "The very fact that they have used chemical weapons without any appropriate reaction is a very worrying development, because it might signal that this is legitimate."

Charges of chemical weapons use come as the U.S. and its allies struggle to find a viable approach to the Syrian conflict, now in its third year.

The White House has been trying to slowly build pressure on Syrian President Bashar Assad, while stopping short of providing military aid to the rebels. Over the weekend, officials pledged an additional $123 million in nonlethal aid.

Yet the rebels remain badly divided, as do their foreign supporters. Worries about extremists in the rebel ranks are causing some of the strongest advocates for the rebels to think twice about how much support to provide.

Even while they allege that Syria used chemical weapons and push for the end to a European Union arms embargo, France and Britain appear to have softened their position, partly out of concern about Islamist extremists in the rebel camp. One of the strongest rebel groups, Al Nusra Front, recently declared its allegiance to Al Qaeda.

Syrian government forces have gradually pulled back to protect Damascus and a handful of major cities that they at least partially control, as well as key military bases they use to project air and artillery power. But reports from inside the country in recent weeks suggest they have gone on the offensive in some areas.

U.S. appeals to Russia to drop its support for Assad have not yielded any visible progress. Secretary of State John F. Kerry told reporters in Brussels on Tuesday that Russian Foreign Minister Sergei Lavrov had agreed to work with him to open negotiations on a new Syrian government.

"I would say to you that it's a very difficult road," Kerry added. "No one should think there is an easy way to move forward on this."

The Obama administration is divided on how much it should do to help the opposition. Obama has declared that chemical weapons use by Assad was a "red line" and a "game changer," to which the United States would unquestionably respond. But he is known to be reluctant to step into the Syria conflict more forcefully.

Kerry urged nervous North Atlantic Treaty Organization allies to begin considering how they would respond if the Syrian regime used chemical weapons.

Kerry told officials of the Western military alliance that they needed to "carefully and collectively consider how NATO is prepared to respond to protect its members from a Syrian threat, including any potential chemical weapons threat." NATO member Turkey has a long border with Syria and is a base for rebel groups.

But Kerry told reporters later that he wasn't calling for more planning for how to respond to possible chemical weapons use.

U.S. officials revealed last week that the Pentagon was sending 200 troops to Jordan, Syria's southern neighbor, the vanguard of what could be a much larger force that could be deployed if the administration concludes it needs to secure Syria's chemical weapons or prevent the war from spilling over the borders.

One Pentagon study estimated it could require 70,000 troops to respond, though other analysts contend that figure is inflated.

Some senior officials like Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, have openly warned about military involvement.